The image hard copying generally relates to the screening and plate-making technology of printers and advanced printing and plate-making equipments. The screening technology used for image hard copying is also called the technology of digital image halftone. The technology of digital image halftone comprises amplitude-modulation screening and frequency-modulation screening. The technology of amplitude-modulation screening is also called ordered dithering of gathered dots, characterizing in that produced colored dots in a halftone image are pairwise gathered geometrically so as to form clusters of colored regions called dots. Since the technology is to control the area of the dots to represent the gray level of the original image, the dots is called amplitude-modulation dots.
On the other hand, the frequency-modulation halftone image avoids the geometrical gather of the colored dots in the process of production of the image. The technology represents the gray level of the original image by controlling the number of the colored dots in a unit area. Since the colored dots in the frequency-modulation halftone image are not distributed in the form of gathering, different gray levels in the original image are corresponding to different average distances among the dots. According to the view of digital image processing, the frequency of the image is variable. Thus, the technology is called frequency-modulation screening.
In the field of printing, the technologies of amplitude-modulation and frequency-modulation have respective advantages and disadvantages for the process of image hard copying. Since the amplitude-modulation dots have certain dimensions (i.e. the halftone count) and specific regular angles (i.e. the halftone angle) for arrangement of the dots, the represented tone of the image is very regular during the practical process of a halftone image, without generating any texture. Thus, the object bearing the printed content looks “clear”. However, in the practical process, when halftone images with different angles and slightly different counts are overlapped, the optical interference phenomenon is generally generated due to the characteristics of the counts and angles. The phenomenon is called “moire” in the printing field. Furthermore, the amplitude-modulation halftone dots also have the characteristic of shape of the dots. Therefore, the shape of the dots will be changed with the change of the size of the dots during the change of continuous tone gradations so as to generate a phenomenon of jump and loss of the continuous tone gradations. The familiar “moire” and phenomenon of jump of gradations should be avoided or inhibited in practical use so that the process is needed to be improved. The conventional amplitude-modulation screening is disclosed in a reference: Screen Techniques: Moire in Four Color Printing, Paul A. Delabastita, TAGA Proceedings, 1992, 1:44-66.
To the contrary, the size of the frequency-modulation halftone dots is changeless (the size generally equals to that of a pixel of the output equipment and the count does not exist) and the distribution of the dots has the characteristic of random dithering (the angle does not exist). Therefore, the “moire” and jump of gradations in the amplitude-modulation dots are avoided completely. However, other problems occur due to the characteristic of distribution of random dots. Since the distribution of the frequency-modulation halftone dots is random absolutely and sharpens the image, the texture due to the random effect generally occurs in those medium gradations of the continuous tone image and “worm” texture generally occurs in light and deep gradations. The texture is hard to be avoided because of the random characteristic. Furthermore, the size of the frequency-modulation halftone dots generally equals to that of a single dot of a physical equipment. Thus, some of the dots are generally lost if the status of the physical output equipment is not good. Further, frequency-modulation screening generally loses gradations especially in the light and deep gradations during the process so that the image cannot be represented with high quality. The conventional error distribution algorithm is disclosed in a reference: Printer Models and Error Diffusion, T. N. Pappas and D. L. Neuho, IEEE Transactions on Image Processing, vol. 4, pp. 66-79, January 1995.
In the field of printing and plate-making, the output equipment used for image hard copying is generally a laser phototypesetting machine, a laser printer, a laser inkjet printer, and the like. Since the two types of halftone dots are limited by the process, the amplitude-modulation dots is mostly used in the field of laser phototypesetting with back-end printing and the frequency-modulation dots is mostly used for equipments in the field of desktop office such as a laser printer, an inkjet printer, and the like. As a whole, the application scope of the frequency-modulation halftone dots is not wider than that of the amplitude-modulation halftone dots because the size of the frequency-modulation halftone dots is changeless and too small. Thus, if the precision of the physical output equipment and the adsorbability of the ink and the object bearing the printed content are not satisfied requirements, the effect of the represented image produced by the frequency-modulation halftone dots is far away from that produced by the amplitude-modulation halftone dots. The reason is that the size of the amplitude-modulation halftone dots is changed in different gradations so as to ensure enough ink is adsorbed on the object bearing the printed content.
Although the effect of the frequency-modulation halftone dots based on the error distribution technology is better than that of amplitude-modulation halftone dots in detail. However, the application of the amplitude-modulation halftone dots is limited because of the above-mentioned disadvantages and it should be improved.